FIGURE 17 Extraocular muscles and cranial nerves responsible for movements of right eye. (A) Extraocular muscles are controlled by three cranial nerves (III, IV, and VI). The levator palpebrae opens the eyelid and is innervated by cranial nerve III. (B) Horizontal gaze is controlled by paired contraction-relaxation of the medial rectus and lateral rectus muscles. (C) Vertical gaze is controlled by four muscles. Downward gaze is produced by inferior rectus and superior oblique muscles. Because of their placement on the eye, these muscles will also cause lateral and rotational movements, as well (indicated by orientation of the radial lines on the iris). Upward gaze is similarly controlled by the superior rectus and inferior oblique muscles.

51. The Visual System also adducts and intorts (i.e., rotates the eye around an anteroposterior axis so that the top of the cornea moves in a circular arc in the nasal direction). The inferior rectus depresses but also adducts and extorts (i.e., the top of the cornea rotates in the temporal direction).

The superior and inferior oblique muscles also contribute to vertical and rotational movement of the eyeball. The superior oblique muscle is innervated by cranial nerve IV, and the inferior oblique is innervated by cranial nerve III. The superior oblique muscle runs from the tendinous ring through the trochlea and attaches to the eyeball 51° off center. It depresses the eye but also abducts and intorts the eye. The inferior oblique muscle originates from a tendinous area near the lacrimal gland and attaches to the eyeball 51° off center. It elevates but also abducts and extorts.

Figure 17 illustrates the primary movements generated by the extrinsic muscles of the eye. Only the right eye is illustrated. Note that cranial nerves III and VI participate in generating horizontal gaze; cranial nerves III and IV participate in generating vertical gaze. The eyelids are elevated by two groups of muscles: the levator palpebrae muscle, which is innervated by cranial nerve III, and the tarsalis muscle, which is innervated by sympathetic fibers. The eyelids are closed by the orbicularis oculi muscle, which is innervated by cranial nerve VII. Figure 18 diagrams the location of cranial nerves III, IV, and VI and their nuclei within the brain stem. It also illustrates certain topographical relationships of these cranial nerves that may play a role in their susceptibility to certain types lesions. For example, pituitary tumors may compress the oculomotor nerve as it exits the brain stem. Also note that the oculomotor nerve passes near the branch points of two major arteries (posterior cerebral and superior cerebellar), sites that are susceptible to aneurysms. The trochlear nerve is somewhat vulnerable to head trauma because of its long projection course around the lateral aspect of the brain stem. The abducens nerve ascends alongside the basilar artery and is vulnerable to vascular insults involving this artery.

Intrinsic Muscles of the Eye

In addition to its somatic motor component, the oculomotor nucleus contains a parasympathetic component called the Edinger-Westphal nucleus. It is located near the midline of the midbrain and sends preganglionic fibers to the ipsilateral ciliary ganglion. Postganglionic fibers innervate the ciliary muscle that controls the shape of the lens. Postganglionic fibers of the ciliary ganglion also innervate the pupillary sphincter muscles that constrict the pupil. Thepupillodilator muscle is innervated by sympathetic neurons.

A. Ventral view of brain stem

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